The invention relates to an electromedical implant, such as for example, a cardiac pacemaker, cardioverter, defibrillator or a pure monitoring implant, which comprises means to detect the impedance of human or animal tissue.
Furthermore, the invention relates to a system comprising at least one such electromedical implant which, in addition, comprises a wireless data interface, and to a patient device, a home monitoring service center or both.
From the state of the art, electromedical implants with means to determine the impedance of human or animal tissue are for example known from U.S. Pat. No. 5,957,861. There, a device is presented which by regular measuring and long-term monitoring of the impedance of pulmonary tissue facilitates early detection of the formation of pulmonary oedema. Oedema are accumulations of fluid, which occur in particular in patients with chronic cardiac insufficiency. The body reacts to the heart's reduced pumping capacity by means of compensation mechanisms which, however, in the long term result in progressive deterioration of the cardiac state. Among other things, the initial load is increased, and there is increased fluid retention. In an advanced stage, the increased pressure in the pulmonary circulation system can cause the formation of pulmonary oedema.
Impedance measuring of the pulmonary tissue utilizes the effect wherein body tissue containing fluid has a conductance which differs from that of healthy tissue. Experience shows that such impedance measuring makes possible earlier diagnosis of a pulmonary oedema than would be possible with conventional methods. The development of electromedical implants, which implants by way of impedance measuring provide such data to a physician which make it possible to diagnose a pulmonary oedema, is therefore attracting considerable attention. In this context, the integration of impedance measuring in a cardiac pacemaker has proven to be particularly advantageous.
Normally, cardiac pacemakers are implanted underneath the clavicle on the side opposite that of the heart, i.e. on the left side when viewed by the physician, and comprise an electrode which is anchored in the cardiac tissue of the patient. In this constellation, most of the patient's pulmonary tissue is located between the cardiac pacemaker housing and the electrode in the heart. If the cardiac electrode then injects a small current that is too weak to cause a heartbeat or some other reaction of the body, then at the same time the voltage that is present between the cardiac pacemaker housing and the electrode can be measured. When related to the value of the initiated current, this voltage is an indicator of the state of the pulmonary tissue of the patient. However, it is not sufficient to consider the absolute value of the measured impedance because the impedance to a large extent depends on the patient's anatomy and on the quality of the contact between the cardiac pacemaker electrode and the body tissue. Instead, the measured impedance values have to be stored for an extended period of time and must be compared with older values. If within days or weeks changes in the measured impedance value occur, this can indicate the formation of a pulmonary oedema.
Commercially available cardiac pacemakers and other electromedical implants usually feature a wireless data interface by way of which the cardiac pacemaker can transmit medical and technical data to an external device. If long-range telemetry has been integrated in the pacemaker, the impedance values of the pulmonary tissue, which impedance values have been determined by such a cardiac pacemaker, can be transmitted in this way to a home monitoring service center where the data is made available to an attendant physician for diagnosis. If on the basis of the transmitted data the attendant physician considers that there is a danger of a pulmonary oedema forming, the patient can be admitted for treatment long before physical discomfort occurs.
However, the device described in U.S. Pat. No. 5,957,861 is associated with a disadvantage in that long-term changes in the measured impedance values can also occur due to other causes, e.g. scarring (cicatrisation) of the tissue surrounding the cardiac pacemaker and the cardiac pacemaker electrode, or a change in the cardiac geometry due to advancing cardiac insufficiency.